Open a graphics window.
The graphics window is cleared and the current point is set
to (0, 0). The string argument is used to pass optional
information on the desired graphics mode, the graphics window
size, and so on. Specification can be found at
http://www.w3schools.com/jsref/met_win_open.asp.
Note: an extra specification is availble, "target",
to specifies the target attribute or the name of the window.

Return the position of the mouse cursor, relative to the
graphics window. If the mouse cursor is outside of the graphics
window, mouse_pos() returns a point outside of the range
0..size_x()-1, 0..size_y()-1.

valbutton_down:unit -> bool Lwt.t

Return true if the mouse button is pressed, false otherwise.

valread_key:unit -> char Lwt.t

Wait for a key to be pressed, and return the corresponding
character. Keypresses are queued.